Zmodell digital decoder for AZL SW1500 switching locomotive

Greetings to all,

I would like to introduce my next development – digital decoder for AZL SW1500 switching diesel locomotive. After successful production of the digital decoder for AZL Alco RS series locomotives, I received numerous requests regarding whether it would be possible to develop a similar solution for upcoming switcher from AZL.

Soon after announcement of SW1500 by AZL back in October, I decided to start a development. It would be impossible without invaluable help and direct assistance from AZL – they sent me pre-production samples and provided a very important technical documentation.

Initial measurements revealed the fact that development of the digital decoder for this small locomotive will be a big challenge: the model is very small and available space inside is extremely limited. It became obvious to me that approach I used in most of my previous developments (soldering ready decoders onto the main circuit board) will not work here.

However, none of these stopped me, and I decided to do what I was already thinking about some time ago. I acquired a license from Doehler & Haass to produce my own decoders using their proprietary chips and decoder firmware. It extended my developing possibilities a lot, as I was not limited anymore to dimensions of existing decoders, types of used components and their arrangement on the circuit board – it became possible for me to develop everything from scratch, according to my own requirements.

There is only approx. 1.1 mm of available height between the chassis and locomotive’s housing, so initial conditions for choosing the type of the circuit board and components are very tough. I had to choose the smallest and thinnest elements ever available. However, my selection was limited by the height of D&H’s proprietary chip that has 0.9 mm height and produced only in a single type of package.

After settling the question of all required components, I had to choose what kind of material I have to use for the circuit board. Doehler & Haass recommends 4-layer design for small decoders in order to provide correct grounding plane and minimal trace length. For example, 4-layer design and 0.5 mm thickness of circuit board material is used in their popular decoder DH05C.

However, I had to avoid 4-layer design at any costs for the following reason – maximal thickness of the circuit board for this decoder should not exceed 0.3 mm. Anything thicker will not allow the housing to sit on the chassis properly. I was not able to find a PCB provider who could produce a rigid 4-layer circuit board only 0.3 mm thick. I don’t know – maybe such manufacturers indeed exist; but I believe such circuit board will cost more than gold.

So, I managed to trace the board in only two layers. It was a very difficult task, since not everything was traced in automatic mode successfully – I had to do the rest manually. I had to use very small vias and only 0.15 mm trace width. Given a very low power consumption of the locomotive, this is fully acceptable.

One important note about PCB thickness: to allow the housing to sit on the chassis completely, I need to reduce the thickness of the circuit board to as low as 0.2 mm. Such materials exist indeed; however, the circuit board based on this material will be too weak to be installed into the locomotive safely. Therefore, I decided to increase the thickness to 0.3 mm. It is also very low, but much better than 0.2 mm in terms of durability.

As a result, the housing of the locomotive will sit 0.1 mm higher after installing the decoder. But actually, the difference will not be visible at all.

Here is the circuit board. Its dimensions are 32.7 mm x 6.7 mm (excluding additional circuit board with rear LED):


This is how it looks when installed onto SW1500 chassis:


The decoder has a full set of features and benefits equal to Doehler & Haass DH05C digital decoder – it supports multiple protocols (DCC, SX1, SX2, MM and DC analog), as well as D&H’s proprietary Super-Soft Drive load regulation technology and firmware updates.

Note: Decoder shown on the pictures is a pre-production version (test sample). I assembled it using a different material with higher thickness – 0.6 mm (in order to save on production costs). Final version will use 0.3 mm material, gold plating instead of conventional tinning and mask with a different color.

I want to express my sincere gratitude to those without whose help this development would not have been possible:

- Doehler & Haass for licensing me their proven and reliable digital decoder technology, as well as for excellent technical support;

- AZL team for providing locomotive samples and important technical documentation;

- Claudius Veit, developer and manufacturer of Velmo decoders – for invaluable support, as well as for help with obtaining some critical electronic components.

Now about availability: I plan to launch a production run in the nearest time. I am not sure if it will be enough time for it in this year, but it will definitely happen in the beginning of 2022. For this, I need to estimate a demand in advance. This decoder is too complex for manual assembly in big quantities; therefore, I plan to order an automatic assembly at one of local providers.

I expect the decoder to be available from selected dealers in the USA; if I find a seller in Germany, I will report it too. Otherwise, all my products are always available from me directly.

PS. One global problem that can potentially slow down the production process – is a world shortage of chips. Unfortunately, this fact fully influences on the train model industry too. One crucial component used in D&H decoders, as well as in this one is a microcontroller from Microchip – PIC16F1825-I/ML. It is currently out of stock everywhere, and lead time is around a year. Although I was lucky to acquire some of these chips recently, I need much more for production run. I would be more than happy if someone could help me with this chip in QFN16 4x4x0.9 mm package or just told me where to find them.


Best regards,
Alex